1) The AA. present a contribution toward the elaboration of a method for the laboratory diagnosis of Chagas' disease, suitable for routine work. 2) Complement fixation test has been studied in detail and attempts to remove the difficulties which prevent the practical application of this test, have been realized. 3) Agglutination test has been performed on acute and chronic cases of Chagas disease as well as in healthy individuals, and results compared with complement-fixation test. 4) The organisms used as antigen in both tests have been, cultured suc¬cessfully in a broth-dextrose-rabbitt blood-agar in Erlenmeyer flasks, co¬vered by a thin layer of liver broth-glucose, as described in the text. In this medium the flagellates have grown luxuriantly and a single Erlenmeyer flask containing only 25 cc. produces more or less 0,5 cc. of packed flagellates. 5) A comparative study of the various antigens (Romaña & Dias, Kelser, Davis etc.) proposed for the complement fixation test led to the follo¬wing conclusions. a) A suspension of the parasites killed and preserved with 0,1% for¬malin, is practically devoid of fixation power. b) Autolysates present poor preservation capacity and its fixative ac¬tivity rapidly falls. c) Dessicated flagellate bodies are easily preserved and fix very well the complement, but is more strongly anti-complementary than Davies anti¬gen. d) Romaña & Dias antigen presents a weak fixative power as compa¬red with Kelser and Davis preparations. e) Kelser antigen when recently prepared, has a highly fixative capa¬city which soon falls, with a progressive increase in its anticomplementary activity. f) Davis antigen is the best. It has a strong fixative power, and a small anticomplementary activity which are not altered by storage, at least for three months, in accordance with AA. experience. This antigen has been adopted in this work. 6. Variations in the fixative capacity of different strains of Schizotry-panum cruzi could not be observed. 7) The complement fixation applied to sera from cases of Chagas' dise¬ase but using an antigen suspension of Leishmania btasiliensis (Davis method) and Trypanosoma equinum (living organisms) has shown a strong fixative property of both antigens, chiefly in tubes with a greater concentration of the serum. A different result was observed in relation to the behaviour of horse serum which gave positive fixation reaction only in the presence specific antigen (Trypanosoma equinum). These observations as well as the action of Chagas disease sera on Leishmania brasiliensis and vice-versa (see tables V and VI), suggests the existence of greater concentration of group antigens in the Schizotrypanum cruzi than in Leishmania btasiliensis and Trypanosoma equinum. 8) Davis method has been tried with Leishmania brasiliensis and Try¬panosoma equinum. With the former, it gaves entirely satisfactory results in the complement fixation test and allergic skin test for diagnosis of cutaneous leishmaniasis...